Expression of a non-membrane bound receptor (soluble) analogue is a common structural motif among members of the hematopoietin superfamily of receptor. Our laboratory identified and cloned a soluble component of the low affinity receptor of the granulocyte- macrophage colony stimulating factor(sGMRalpha). The proposed studies aim to characterize sGMRalpha to gain insight into the biological function of a class of proteins that are highly conserved through evolution. Previous studies in the laboratory suggested that sGMRalpha can inhibit GM-CSF binding to its receptor. I developed a reverse transcription-polymerase chain reaction assay to define the biology of sGMRalpha expression. Using information derived from this assay, experiments are proposed that critically test the hypothesis that sGMRalpha is a specific inhibitor of GM-CSF receptor function. These experiments seek to determine the role sGMRalpha binding of GM-CSF in the extracellular space to prevent it from activating the membrane-associated receptor and whether sGMRalpha directly interacts with the GM-CSF receptor complex. The effect of sGMRalpha on the biological endpoints of cell proliferation, differentiation, and degranulation will be determined. In addition, sGMRalpha effects on proximal pathways of GM-CSF-mediated signal transduction will be analyzed. Site-directed mutagenesis will delineated the regions of critical importance in sGMRalpha function. Since a soluble, specific inhibitor of -csf action may be developed as a potential antineoplastic or antiinflammatory agent, the characterization of sGMRalpha has considerable clinical significance.